Visualizations of volumetric data sets can be generated using different 3-D projection models, such as, for example, an orthographic projection or a perspective projection. In a perspective model, the on-screen size of the volumetric data depends on the depth position, i.e., the farther the data is into the depth of the virtual projection model used for rendering, the smaller it is depicted. See, Perspective Volume Rendering, D H Porter, 1991, University of Minnesota Supercomputer Institute Research Report UMSI 91/149. A projection, preserving size along the depth dimension, is known as an orthographic projection. FIGS. 1A and 1B illustrate examples of projections of volumetric data consisting of two equal-size objects onto a screen. FIG. 1A is an orthographic projection. FIG. 1B is a perspective projection, where the far-away object is rendered smaller. The perspective model is typically considered advantageous for human interpretation of a visualization as the human visual system is accustomed to connecting perspective to depth cues.
There are many possible ways to depict volumetric data. One of the important benefits of using 3-D visualizations compared to, for instance, 2-D slices is the level of realism achieved. A 3-D depiction of medical data resembles looking at the patient in reality and can therefore be an easily understandable visualization.